Conventional aircraft environmental control systems incorporate a turbomachine, commonly referred to as an air cycle machine or an air cycle cooling machine, for use in cooling and dehumidifying air for supply to the aircraft cabin for occupant comfort. Such air cycle machines may comprise two, three or four wheels, including at least one compressor and at least one turbine, disposed at axially spaced intervals along a common shaft. The turbine or turbines, as the case may be, extract energy from a working fluid passing therethrough for driving the compressor and any other wheels mounted to the shaft, such as a fan or an additional compressor. The three wheels are supported for rotation about the axis of the shaft on one or more bearing assemblies disposed about the drive shaft, such as a pair of spaced bearing assemblies, one bearing assembly disposed intermediate each pair of adjacent spaced wheels. Although the bearing assemblies may be ball bearings or the like, hydrodynamic film bearings, such as gas film foil bearings, are often utilized on state-of-the-art air cycle machines.
On aircraft powered by turbine engines, the air to be conditioned in the air cycle machine is typically compressed air bled from one or more of the compressor stages of the turbine engine. In conventional systems, this bleed air is passed through the air cycle machine compressor wherein it is further compressed, thence passed through a condensing heat exchanger to cool the compressed air sufficiently to condense moisture therefrom thereby dehumidifying the air before expanding the dehumidified compressed air in the turbine or turbines, as the case may be, of the air cycle machine to both extract energy from the compressed air so as to drive the shaft and also to cool the expanded turbine exhaust air for use as the cooling fluid in the aforementioned condensing heat exchanger before it is supplied to the cabin as conditioned cooling air. The compressed bleed air being supplied to the compressor of the air cycle machine is typically precooled by passing it through a precooling heat exchanger in heat exchange relationship with ambient air drawn through the precooling heat exchanger by the fan of the air cycle machine.
As aircraft environmental control systems must be capable of delivering conditioned air to the aircraft passenger and crew cabins over a wide range of operating conditions, it is necessary during portions of the operational envelope to bypass a portion of the working fluid bled to the system about the turbine, or at least one turbine in multi-stage turbine machines, and thereafter readmit the bypass fluid with the turbine exhaust, i.e. that portion of the working fluid passed through the turbine. For example, in commonly assigned U.S. Pat. No. 4,430,867, there is disclosed an air cycle refrigeration system for cooling and ventilating an aircraft enclosure wherein provision is made for bypassing a portion of the system working fluid, i.e. compressed air bled from the aircraft engine compressor, about the turbine of a three wheel air cycle machine under certain operating conditions, and thereafter readmitting this bypass air into the turbine exhaust being directed to the aircraft enclosure at a point downstream of the turbine. In this system, a portion of relatively warm air may be bypassed around both the compressor and turbine of the air cycle machine and subsequently remixed therewith at a location downstream of the turbine and upstream of a cabin air recirculation heat exchanger so as to raise the temperature of the mixed air stream sufficiently to prevent icing in the cabin air recirculation heat exchanger.
The environmental control system disclosed in commonly assigned U.S. Pat. No. 5,086,622 operates in a condensing cycle mode, employing a four wheel air cycle having a first stage turbine and a second stage turbine, which together serve to extract energy from the system working fluid, again engine compressor bleed air, and condition the working fluid for supply to the aircraft enclosure. Water vapor is removed from the compressed air before it is expanded in the first turbine by passing the incoming air through a condensing heat exchanger in heat exchange relationship with the chilled exhaust from the first turbine so as to condense moisture from the incoming air. After passing through the condensing heat exchanger, the first turbine exhaust, having been warmed via absorption of the heat given up by the condensing water vapor, is expanded through the second turbine. Under certain operating conditions, a portion of the working fluid is bypassed about the second turbine and thereafter remixed with the exhaust from the second turbine being directed to the aircraft enclosure at a location downstream of the second turbine. Commonly, the relatively warm bypass air has been perpendicularly introduced into the chilled turbine exhaust downstream of the turbine outlet so as to create flow turbulence in order to facilitate mixing of the bypass air with the turbine exhaust air. While effective in improving mixing, such perpendicular bypass air introduction is often accompanied by undesirable noise generated by the flow turbulence during the mixing of the flow streams.
Under some operating conditions, the turbine exhaust may be cooled to such a point that residual moisture remaining in air supplied thereto condenses on the walls of the turbine outlet shroud and outlet duct work upstream of the location of the admission of the turbine bypass air, thereby causing undesirable icing on such upstream surfaces. One method of addressing icing of the turbine outlet shroud and downstream exhaust ductwork is to pass a flow of relatively warm air over the external surfaces of the shroud and the turbine exhaust ductwork. For example, U.S. Pat. No. 4,580,406 discloses an air cycle environmental control system wherein a first portion of the relatively warm bleed air being supplied to the compressor of the air cycle machine is first passed over the outside of the turbine outlet shroud to heat the shroud before being supplied to the air cycle machine compressor. A second portion of the relatively warm bleed air being supplied to the compressor of the air cycle machine bypasses the air cycle machine altogether and is passed into a plenum chamber disposed about the turbine exhaust duct, thereby heating the outside surface of the wall of the turbine exhaust downstream. The bypass air admitted to this plenum chamber is vented therefrom into the turbine exhaust air immediately upstream of a condensing heat exchanger disposed downstream of the turbine outlet through which the chilled turbine exhaust passes in heat exchange relationship with the compressed bleed air being passed to the turbine for expansion therein.